The present invention relates to a voltage conversion circuit that converts an input voltage, a strobe device including the voltage conversion circuit, a photographing device including the strobe device, and a method of reducing a surge voltage in the voltage conversion circuit.
In general, a strobe device including a boosting circuit having a boosting transformer and a switching device, a capacitor and a Xenon tube is used as an illuminating device for a photographing device, such as a camera. In recent years, in accordance with downsizing of cameras, a demand for downsizing the strobe device is also increasing. Regarding a boosting transformer of the strobe device, the switching frequency thereof is a low level of 2 kHz to 100 kHz regardless of the fact that the boosting ratio is a large level of 15 to 30 times. Therefore, if inductance of the primary side of at least 10 μH to 20 μH is not secured, it becomes difficult to keep transition of an inrush current at the time of start of switching within a controllable range. If the number of turns of the primary side of the boosting transformer is set to approximately 15 turns to secure such inductance of the primary side, the number of turns of the secondary side increases to 225 to 450 turns and thereby the size of the boosting transformer increases.
If a diameter of a used wiring member of the boosting transformer is decreased to downsize the boosting transformer, a cross sectional area of the wiring member decreases and thereby a direct current resistance component of the wiring member increases. However, since charge of the capacitor needs to be completed within a predetermined time period, an electrification current amount from the start of charge to the end of charge cannot be decreased. Therefore, heat generation by the copper loss increases, the temperature of the boosting transformer itself becomes high, and thereby the boosting efficiency may deteriorate by inducing magnetic saturation. Furthermore, there is a possibility that internal short-circuit in the boosting transformer is caused by melting of the isolation coating of a coil of the boosting transformer due to thermal storage and thereby the boosting transformer becomes unable to function. As described above, in order to downsize the strobe device, it becomes an important point to prevent occurrence of failure caused by heat generation by the boosting transformer.
The major part of cameras in recent years have both the function of capturing a still image and the function of capturing a moving image, and are provided, as a standard function, with the function of constantly displaying a moving image on a liquid crystal display device for video in place of an optical finder. In a camera which does not constantly display a moving image, the charging operation for charging a capacitor of a strobe device and another function are controlled to be performed exclusively. On the other hand, in a camera which constantly displays a moving image, it becomes necessary to control the moving image displaying function and the charging operation for charging the capacitor of the strobe device to be performed concurrently. A problem which would occur in such a situation is that a large amount of switching current caused during the charging affects a circuit which is vulnerable to a magnetic noise leaking from the boosting transformer and thereby noise is superimposed on a moving image. Further, there is a possibility that, since a large amount of current ripple is actually caused on a power line, a power input and a GND level of an electronic circuit are made unstable and thereby noise affecting the entire circuit is caused.
As a method for solving the above described problem, a method for increasing the number of transferring times of current per a unit of time while increasing the switching frequency and decreasing the electrification current amount per one time is known. When such a method is applied to a strobe device, at least inductance on the primary side can be decreased to approximately 0.5 μH to 1 μH by increasing the switching frequency to 300 kHz to 1 MHz. Therefore, in this case, the number of turns on the primary side becomes approximately 5 turns, and the number of turns on the secondary side can also be decreased to 75 to 150 turns. As a result, the boosting transformer can be downsized. Furthermore, since the number of turns can be decreased, downsizing can be realized and the diameter of the wiring line can be increased. Therefore, heat generation by the copper loss can be suppressed to a low level. Furthermore, since the electrification current amount per one switching is decreased, downsizing of peripheral components of the transformer, such as a rectifying diode and the switching device, can also be achieved simultaneously, and thereby the cost down of the entire strobe device can be expected.
However, when the transformer is downsized to some extent or more and the number of turns on the primary side is decreased, magnetic coupling between the transformer and a magnetic circuit, such as a ferrite, constituting a core, becomes sparse, and thereby the leakage magnetic flux of the primary coil becomes relatively large. In this case, various effects may be caused to the circuit. Japanese Patent Provisional Publication No. 2005-317278A (hereafter, referred to as patent document 1) suggests technology where a charged voltage of a capacitor is detected in a strobe device while considering effect by a leakage magnetic flux. The strobe device described in patent document 1 detects the primary voltage inducted on the primary side of the transformer during boosting and judges whether the charge of the capacitor is completed based on the detected primary voltage. In this case, in order to decrease the effect of the noise by the leakage magnetic flux on the primary side, the strobe device is configured to accurately detect the voltage of the capacitor by latching the primary voltage after a predetermined time period has elapsed and thereby the noise level becomes small. As a result, a downsized and inexpensive strobe device can be realized.